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CHAPTER 4
ALTERNATING CURRENT MOTORS
LEARNING OBJECTIVES
Upon completion of this chapter you will be able to:
1. List three basic types of ac motors and describe the characteristics of each type.
2. Describe the characteristics of a series motor that enable it to be used as a universal motor.
3. Explain the relationships of the individual phases of multiphase voltages as they produce rotating
magnetic fields in ac motors.
4. Describe the placement of stator windings in two-phase, ac motors using rotating fields.
5. List the similarities and differences between the stator windings of two-phase and three-phase ac
motors.
6. State the primary application of synchronous motors, and explain the characteristics that make
them suitable for that application.
7. Describe the features that make the ac induction motor the most widely used of electric motors.
8. Describe the difference between the rotating field of multiphase motors and the "apparent"
rotating field of single-phase motors.
9. Explain the operation of split-phase windings in single-phase ac induction motors.
10. Describe the effects of shaded poles in single-phase, ac induction motors.
INTRODUCTION
Most of the power-generating systems, ashore and afloat, produce ac. For this reason a majority of
the motors used throughout the Navy are designed to operate on ac. There are other advantages in the use
of ac motors besides the wide availability of ac power. In general, ac motors cost less than dc motors.
Some types of ac motors do not use brushes and commutators. This eliminates many problems of
maintenance and wear. It also eliminates the problem of dangerous sparking.
An ac motor is particularly well suited for constant-speed applications. This is because its speed is
determined by the frequency of the ac voltage applied to the motor terminals.
The dc motor is better suited than an ac motor for some uses, such as those that require variable-
speeds. An ac motor can also be made with variable speed characteristics but only within certain limits.
Industry builds ac motors in different sizes, shapes, and ratings for many different types of jobs.
These motors are designed for use with either polyphase or single-phase power systems. It is not possible
here to cover all aspects of the subject of ac motors. Only the principles of the most commonly used types
are dealt with in this chapter.
